Association between ambient air pollution and colorectal cancer risk: The Multiethnic Cohort Study

Growing evidence links air pollution to colorectal cancer (CRC) incidence. We examined this association within the large Multiethnic Cohort Study (MEC). Geocoded residential addresses for 98,675 California MEC participants were appended to ambient air pollution measures of PM_2.5 (particulate matter [PM] with an aerodynamic diameter <2.5 μm), PM₁₀ (PM < 10 μm), nitrogen dioxide (NO₂), nitrogen oxides (NO_x), carbon monoxide (CO), and ozone (O₃), generated from enrollment (1993-1996) to December 31, 2018. Multivariable-adjusted Cox proportional hazards models evaluated associations of time-varying air pollutants with CRC incidence (n = 3217 cases). We assessed heterogeneity in associations by demographics, tumor stage, and anatomical subsite. CRC incidence increased with PM_2.5 exposure (per 10 μg/m³; hazard ratio [HR] = 1.13, 95% confidence interval [CI] = 0.96-1.33), mainly among female (HR = 1.29, 95% CI = 1.03-1.62) but not among male participants (P_{heterogeneity} = 0.08). CRC incidence also increased with NO_x exposure among female (HR = 1.22, 95% CI = 1.01-1.48) but not male participants (P_{heterogeneity} = 0.07). Increased incidence associated with PM_2.5 (HR = 1.36, 95% CI = 1.05-1.76), NO₂ (per 20 parts per billion [ppb]; HR = 1.32, 95% CI = 1.05-1.68) and CO (per 1000 ppb; HR = 1.36, 95% CI = 1.01-1.84) exposures were observed for left colon and rectal cancers combined, but not right colon cancers (P_{heterogeneity} by site = 0.08, 0.06 and 0.13, respectively). Associations of PM_2.5 and NO₂ with rectal cancer incidence differed by population group (P_{heterogeneity} = 0.04 and 0.03, respectively), and was mostly driven by positive associations among Latino participants. In summary, increasing PM_2.5, NO₂, NO_x, and CO exposures were suggestively associated with increased CRC incidence, particularly among female and Latino participants and for left colon and rectal cancers.